1. Field
Methods of forming carbon coatings, a composite metal oxide coated with carbon, and/or a lithium ion battery including the same are disclosed.
2. Description of the Related Art
Lithium ion batteries (LiB) typically have high energy density and may be easily fabricated into various designs, and thus have been widely used as an electricity source for portable devices. Recently, as the application of LiB has been extended to power supplies for an electric vehicle and for electrical power storage portable devices, vigorous research has been made on materials that make it possible to realize high energy density and a long battery life-span. One of methods improving battery performance is coating the surface of a composite metal oxide, for example, a positive electrode active material.
In order to improve battery performances in many respects, various coating materials have been proposed. Metal oxides and metal fluorides such as Al2O3, MgO, and AlF3 are examples of such coating materials. Additives capable of forming a film on the electrode surface may play a role in reducing or preventing an electrolyte from directly contacting the surface of the positive electrode, and thereby suppress side reactions caused by electrons transferred therefrom. Such additives may also play a role in eliminating side products that are generated during the charge/discharge cycle in the battery, and thereby impair cell performance. In particular, a carbon coating may improve conductivity of a composite oxide material (e.g., a positive electrode active material), which has an otherwise low conductivity. With the exception of phosphoric acid-based positive electrode materials, however, oxide-based positive electrode materials present difficulties in having a carbon coating thereon due to a CO2 generating reaction that occurs during carbonization.
Recently, growing demand for high energy density have led to a greater need for high voltage positive electrode active materials. However, under a high voltage circumstance, electrolytes are particularly susceptible to oxidation on the surface of a positive electrode active material (i.e., a composite metal oxide material), and thus there remains an urgent need to develop a coating material that may suppress oxidation in the high voltage positive electrode active material. On the other hand, the high voltage positive electrode active material usually includes more lithium and thus has a low level of conductivity, resulting in poor battery performance. In addition, unlike batteries for small and mobile devices, batteries for electric vehicles and electrical power storages are operated and/or kept under such a high-temperature atmosphere and their charge/discharge proceeds so fast that the battery temperature is apt to increase. Therefore, the batteries for electric vehicles/electrical power storages should be able to properly operate even at high temperatures and are desired to have a high level of energy storage efficiency.
In order to improve properties (e.g., battery performance), various attempts were made to form a carbon coating on the surface of the lithium transition metal composite oxide, for example, a positive electrode active material for a lithium ion battery. However, in most conventional attempts, organic materials capable of providing a carboneceous material via thermal decomposition (i.e., a carbon organic precursor) are mixed with a composite metal oxide (e.g., positive electrode active material) or a precursor thereof and then heat-treated to create a carbon coating on the surface of the composite metal oxide. According to such conventional methods, when the mixture of the carbon organic precursor and the oxide-based positive electrode material is heat-treated, the positive electrode active material may suffer an oxygen elimination reaction and thereby has a lower capacity. Moreover, the positive electrode active material may hardly have a uniform carbon coating thereon, and particularly in case of the active material with pores, the inner surfaces of the pores may not have carbon coatings. On the other hand, in order to obtain a crystalline carbon coat having high conductivity, a heat treatment may have to be conducted at a higher temperature, but such a high temperature may cause structural deformation of the active material, and this may lead to a loss of the battery characteristics.
In order for a lithium ion battery to be applied in electrical power storages/electric vehicles, the positive electrode active material may have a high capacity. To this end, for example, use of a positive electrode active material including a higher amount of lithium such as an overlithiated oxide (OLO) has been suggested, but the electron conductivity of the positive electrode active material is too low to obtain good results in terms of a battery life span and a charge/discharge rate. The carbon coating as supplied onto the OLO-based lithium positive electrode active material may be expected to improve the conductivity, but the OLO having carbon coatings applied thereon according to the conventional methods is apt to suffer severe deformation of its crystalline structure so that it becomes useless as a positive electrode active material.
In addition, when the battery is subjected to a charging/discharging process at a high voltage in order to increase capacity, the electrolyte may be easily decomposed on the cathode surface, and the metal component of the active material on the cathode surface may tend to be dissolved into the electrolyte and the dissolved salts may undergo electro-deposition again. Such side reactions on the surface may lead to self-discharge when the battery is stored at a high temperature and they may result in the decrease of the capacity when the battery is charged/discharged at a high temperature.